Known in the art is a method for treatment of products in gas-discharge plasma, said discharge being established in the working space between the anode and cathode under a reduced reaction gas pressure, comprising preheating of the product to working temperature and holding it at temperatures within a preset range (cf. "Chemical heat treatment of materials" by Yu. Lakhtin et al., 1985, Metallurgia PH, Moscow, pp. 177-181 (in Russian).
In the method mentioned above gas-discharge plasma is built up with the aid of a glow discharge at a partial pressure of the reaction gas in the range of 10 to 1000 Pa. Used as the reaction gas most commonly is nitrogen, therefore the method is called ionic nitriding. The process of ionic nitriding involves initiation of an anomalous glow discharge between the product (cathode and the anode, the interelectrode voltage being from 400 to 1000 V. Virtually the entire potential drop in a glow discharge is concentrated at the cathode in the region of cathode potential drop. The ions of the reaction gas (nitrogen) are accelerated in the region of cathode potential drop so as to bombard the surface of the product under treatment, thus heating said surface and simultaneously diffusing depthward to form a hardened superficial layer.
However, bombardment of the surface of the product under treatment with high-energy ions of the reaction gas carried out during ionchemical treatment results in the so-called cathode sputtering of the surface of the product being treated and hence in deterioration of the initial quality of surface finish.
Inasmuch as the process is conducted at a relatively high voltage (400-1000 V) applied to the product being treated, glow discharge is likely to turn into arc discharge (which is most possible to occur at the initial stages of the process). Cathode spots of arc discharge cause erosion of the surface of the product being treated in still higher degree than cathode sputtering. Arcing is reduced by gradual conducting of the process (that is, by reducing the discharge current and voltage). This measure, however, affects throughput capacity. A device for carrying said method into effect and aimed at treatment of products in gas-discharge plasma is known to comprise a direct current source electrically connected to the cathode and anode (cf. "Chemical heat treatment of materials" by Yu. M. Lakhtin et al., 1985, Metallurgia PH, Moscow, pp. 177-181 (in Russian).
Used as the anode in said device is a chamber in which the product being treated is placed, while the cathode is said product itself.
The direct current source enables the voltage to be infinitely adjusted within 1000 V.
The working chamber communicates with a pump to build up vacuum and with the source of the reaction gas which establishes a pressure of 10 to 1000 Pa in the chamber.
Once a voltage has been applied to the electrodes, i.e., the cathode and anode, a glow discharge is initiated in the chamber, whereby the product (cathode) is subjected to bombardment with the ions of the reaction gas. As a result, the product is heated and its surface is saturated with the ions of the reaction gas, thus hardening the surface layer of the product. However, ion bombardment sputters away the surface layer and hence deteriorates the initial quality of surface finish.
One more state-of-the-art method for treatment of products in the plasma of a gas-discharge established between the anode and cathode at a reduced reaction gas pressure is known to comprise preheating of the product to working temperature and holding it at temperatures within a preset range (FI, A, 63, 783).
According to said method, gas-discharge plasma is established with the use of a glow discharge.
The method comprises preheating of the product (cathode) placed in the working chamber (anode) containing the reaction gas (a nitrogen-oxygen mixture) to 400.degree.-580.degree. C., followed by holding the product at said temperature. In view of intensifying the preheating process and increasing the microhardness of the diffusion layer, chemical heat treatment is carried out at a pressure from 0.13 to 13.0 Pa, and the glow discharge is intensified by electrons accelerated to 200 eV.
Since said method is carried out in glow-discharge plasma and the product being treated serves as the cathode, its surface subjected to ion bombardment is sputtered away which affects adversely the initial quality of surface finish.
Still more method for treatment of products in gas-discharge plasma is known to comprise initiation of a vacuum-arc discharge between the anode and the integrally cold cathode, a vacuum-plasma treatment of the product by heating it to working temperature and holding the product in a preset temperature range in the medium of the reaction gas (U.S. Pat. No. 4,734,178).
According to said method, gas-discharge plasma is built up with the use of a more powerful (compared with glow discharge) vacuum-arc discharge involving an integrally cold cathode. A peculiar feature of this method resides in that the product under treatment heated by being bombarded with metal ions. However, treatment of heavy-weight products involves a prolonged heating time which is enough for the product surface is sputtered away, thus deteriorating the initial quality of surface finish.
Moreover, it is due to a low efficiency of the heating procedure that only a relatively low-weight product can be treated, this being on account of a discordance between the time of an optimum radiation dose and the heating time in case of treating heavy-weight products, which narrows the process capabilities of the method as a whole.
A device for treatment of products in gas-discharge plasma is known to carry out the method discussed before, said device comprising: a source of direct current electrically connected to an integrally cold cathode and to an anode, both being enclosed in a vacuum chamber containing the reaction gas at a reduced pressure (U.S. Pat. No. 4,734,178).
In the device mentioned above the product under treatment is heated by metal ions generated by the cathode, which results in sputtering away the surface of the product and hence in a deteriorated initial quality of surface finish.
In addition, said device cannot be used for all-over treatment of products, nor can it perform efficient treatment of dielectric products, which to a great extent restricts its technological capacities.